A New Probe for Long-Lived Particles at Higgs Factories: Displaced Photons in the Hadronic Calorimeter

TL;DR

This work introduces the HCAL-γ_d signature, using the hadronic calorimeter as a far detector for displaced photons from LLP decays at future e+e- colliders. By exploiting detector hermeticity and high-granularity HCAL with particle-flow reconstruction, it turns prompt background suppression into a near background-free search, enabling sensitivity to photon-portal LLPs across a wide range of lifetimes from $L_D$ ~ 1 m to $10^6 R_{in}^H$. In a dark-axion portal benchmark with $m_a = 0.85 m_{γ'}$, the HCAL-γ_d channel yields 10–20× improved sensitivity over conventional ECAL channels in the intermediate decay-length regime $R_{in}^H \lesssim L_D \lesssim 10^6 R_{in}^H$, with ECAL-γ_d and ECAL-γ_r providing complementary coverage for shorter and ultra-long lifetimes. The results highlight a transformative detector-level strategy for LLP searches at Higgs factories and motivate optimized HCAL designs and broader application to other experiments.

Abstract

The search for dark matter and other photon-portal long-lived particles (LLPs) at electron-positron colliders often relies on the mono-photon signature. At future Higgs factories operating at the $Z$-pole, this approach faces a critical challenge: the irreducible background from $e^+e^- \to ν\barνγ$ becomes overwhelming. We propose a novel strategy that overcomes this limitation by searching for displaced photons from LLP decays within the barrel of the hadronic calorimeter. This signature exploits the architectural shielding of the detector to create a nearly background-free environment. Our analysis demonstrates exceptional sensitivity to LLPs with decay lengths from $\sim$1 to $10^6$ meters, improving upon conventional searches by up to one order of magnitude for benchmark photon-portal models.

Figures (4)

• Figure 1: Feynman diagrams of the production (left) and decay (right) processes of photon-portal DS particles at Higgs factories, leading to the mono-photon signature.
• Figure 2: Schematic diagrams for the three mono-photon signatures at CEPC Reference Detector, including (a) HCAL-$\gamma_d$ signature, (b) ECAL-$\gamma_d$ signature, and (c) ECAL-$\gamma_r$ signature, where $\gamma_r$ and $\gamma_d$ denote photons generated by initial state radiation and the radiative decay of DS particles, respectively. Here, $X_2$ and $X_1$ are DS particles generated by $e^+ e^- \to X_2 X_1 (\gamma_r)$, with subsequent decay of $X_1 \to X_2 \gamma_d$.
• Figure 3: Projected $2\sigma$ sensitivity contours for the axion portal model at the CEPC $Z$-mode run. (a) Contours in the plane of coupling ($g_B$) versus dark photon mass ($m_{\gamma^\prime}$), with the axion mass fixed at $m_a = 0.85 m_{\gamma^\prime}$. The sensitivities are shown for several mono-photon signatures, including ECAL-$\gamma_d$, ECAL-$\gamma_r$, and HCAL-$\gamma_d$. For the HCAL-$\gamma_d$ signature, the solid (dashed) curves correspond to conservative (optimistic) background estimates with $N_b = 10^4$ ($10^2$). Gray dashed lines indicate reference decay length $L_D = R_{\rm in}^H$ and $L_D = 10^6\, R_{\rm in}^H$ for the dark photon in the laboratory frame. (b) Contours as a function of mass-splitting parameter $r_{\Delta m } \equiv (m_{\gamma'}-m_a)/m_{\gamma'}$ for fixed dark photon mass $m_{\gamma^\prime} = 0.1\,\rm GeV$.
• Figure 4: The expected $2\sigma$ sensitivities to the neutrino dipole portal for the CEPC $Z$-mode for various mono-photon signatures, including the ECAL-$\gamma_d$, the ECAL-$\gamma_r$, and the HCAL-$\gamma_d$ signatures. For the HCAL-$\gamma_d$ signature, the solid (dashed) line represents the conservative (optimistic) estimate with $N_b = 10^4$ ($10^2$). The gray dashed lines indicate the parameter space with $L_D = R_{\rm in}^H$ and $L_D = 10^6\, R_{\rm in}^H$ for the sterile neutrino in the laboratory frame.